Modern communications systems, such as cell phone services, are heavily reliant on a precise timing reference. Typically, this is provided by a DPLL locked to a primary reference, such as a GPS signal. Occasionally the primary reference signal will be lost, in which case the DPLL goes into holdover mode wherein it is reliant on a precise local reference, which is typically an oven-controlled crystal oscillator (OXCO).
While oven-controlled crystal oscillators provide an extremely stable reference frequency, they are subject to drift over time due to variations in temperature and aging of the device.
U.S. Pat. No. 6,711,230, and the paper entitled Adaptive OCXO Drift Correction Algorithm, by C. W. T Nicholls and G. C. Carleton, given at the 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50th Anniversary Conference, the contents of which are herein incorporated by reference, describe a compensation circuit employing separate Kalman filters that are trained during normal PLL operation, and wherein the trained Kalman filters are used to control the output frequency of the oscillator in the holdover mode based on updates obtained during normal mode. The Kalman filters described in Nicholls assume that the frequency dependence on age is linear, or at least can be approximated by a linear model, with any departure from linearity being corrected by employing higher order polynomial terms within the Kalman filter.
Kalman filters are well known in the art and are typically used to extract data from a noisy environment where it is possible to model the physical behavior of a system. Unfortunately, the frequency dependence of an OXCO with age appears be logarithmic, and is thus not properly compensated over time.